Composite x-ray tube/transformer assembly

ABSTRACT

An elongated X-ray generating tube including a high voltage terminal is disposed in tandem with a high voltage transformer located proximate to the tube and axially aligned therewith. The high voltage terminal is interconnected with the transformer core located centrally of the transformer. An enclosure disposed around the transformer and tube includes a window for transmitting X-rays therethrough while the transformer and tube are embedded or potted as a unit within the enclosure.

United States Patent [191 Gralenski [111 3,812,366 May 21, 1974 COMPOSITE X-RAY TUBE/TRANSFORMER Primary Examiner-Roy Lake ASSEMBLY Assistant Examiner-Darwin R. Hostetter Attorney, Agent, or Firm-Flehr, Hohbach, Test, A]- [75] Inventor. Nicholas M. Gralenski, Aptos, Calif. britton & Herbert [73] Assignee: Watkins-Johnson Company, Palo lt 'l' 22 H d a S g 57 ABSTRACT 1 1e an An elongated X-ray generating tube including a high Pl N03 122,040 voltage terminal is disposed in tandem with a high voltage transformer located proximate to the tube and 52 us. Cl. 250/421 313/55 axially aligned therewith The high terminal is [51] Int. Cl HdSg 1/06 interconnected with the transformer core located 58 Field of Search 313/55 250/90 "ally of the transform enclmure disPOSed around the transformer and tube includes a window [56] References Cited for transmitting X-rays therethrough while the transformer and tube are embedded or potted as a unit UNITED STATES PATENTS within the enclosure 2,925,499 2/1960 Seidel 250/90 5 Claims, 9 Drawing Figures 11 l 2A IO 22 a4 56 r II/I/l/I/l IIIIIIIIIIIIII IIIIII VII/ 1 I6 32 32 L in 29 11 59 17b I11: H

IllIjlI/IIIII/III/Il/II/I/ll/ll/II/Il/Il/I/l COMPOSITE X-RAY TUBE/TRANSFORMER ASSEMBLY BACKGROUND OF TI-IEINVENTION This invention pertains to a composite X-ray generating tube and transformer unit.

The high voltages applied to X-ray tubes are generally considered difficult to handle. Heavy transmission lines, bulky connectors, containers of carefully cleaned and dehydrated transformer oil or high pressure containers of sulfur hexafluoride insulating gas are common place. Changing an X-ray tube requires special skill and equipment to replace the high voltage insulation material. A tube/transformer assembly of the kind described herein, however, contains the entire high voltage circuit and can be handled like any other low voltage component.

Accordingly, a tube/transformer assembly of the type described herein can be changed and serviced much easier than a tube alone.

Further, the usual transformer unit employed with typical X-ray tubes constitutes a somewhat bulky structure necessitating extremely high voltage transmission lines posing various problems of the kind described above and otherwise.

SUMMARY OF THE INVENTION AND OBJECTS In general, a composite unit combining an X-ray generating tube and a high voltage transformer have been provided wherein the high voltage terminal of the tube is disposed proximate to the transformer and means are provided for interconnecting the high voltage terminal to the transformer. Further, means forming an enclosure around the transformer and tube plus shielding means disposed about the tube and including a window for transmitting X-rays therethrough have been provided so as to provide a compact unitary structure. Finally, means supporting the tube and transformer lodged as a unitary structure within the enclosure serves to retain the physical relationship of the transformer and tube in closely proximate relation.

In general, it is an object of the present invention to provide an improved X-ray tube and transformer unit overcoming the foregoing and other problems.

The foregoing and other objects of the invention will become readily evident from the detailed description of preferred embodiments according to the invention when considered in conjunction with the following drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic side elevation section view showing an X-ray tube/transformer assembly according to the invention;

FIG. 2 is a diagrammatic perspective view showing an improved transformer as incorporated in FIG. 1 and in enlarged detail means for electrically interconnecting the transformer and tube;

FIG. 2A represents a section through FIG. 2;

FIG. 3 shows another embodiment of the invention according to FIG. 1;

FIG. 4 is a side elevation view of an X-ray tube of a type suitable for use in conjunction with the embodiment shown in FIG. 3 and (with the modifications made as shown in phantom lines) suitable for use in conjunction with the embodiment shown in FIG. 1;

FIG. 5 is a transverse action view taken along the line 5-5 of FIG. 4;

FIG. 6 is a transverse section view taken along the line 66 of FIG. 4;

FIG. 7 is a diagrammatic side elevation section view showing the relationship between cathode and grid of the tube shown in FIG. 4; and

FIG. 8 is a further enlarged detail view taken along the line 8-8 of FIG. 7 diagrammatically showing the relationship between the cathode and grid.

DESCRIPTION OF PREFERRED EMBODIMENTS As shown in FIG. 1, a composite unit 10 of the kind described comprises an elongated X-ray generating tube 11 including a high voltage terminal 12 and a socalled target assembly 13 arranged to be bombarded by electrons from an electron gun as shown and described with respect to FIG. 4 for the purpose of generating X-rays within the tube 11. Electrical leads 14 are interconnected to operate the electrical portions of tube 11 as described further below.

A high voltage transformer 16 is formed with a coil 17 consisting of primary and secondary winding, each represented diagrammatically merely as a single winding, 17a, 17b respectively, even though the secondary (and sometimes the primary) will usually be constituted of a number of layers of wire. windings 17a, 17b are located respectively on the outer and inner radial portions of the coil 17. As thus arranged, it will be readily evident that the low voltage portion of .the transformer will be disposed on the exterior surface whereas the high voltage portion will be disposed radially adjacent and nearer the core 18 of transformer 16.

Core 18 consists of a short rod-like element and as arranged is much better suited to handling high voltage than other types of transformers. The high voltage in the secondary is developed inwardly with the core element being at high voltage. The core ends may be rounded as shown in FIG. 1 to minimize corona effects. The high voltage is in the center of this configuration where it can be most easily handled as by providing a protruding terminal 19. Tube 11 and transformer 16 are easy to align axially of each other as shown and are embedded in a body of insulating material, such as the plastic potting material 21, for supporting the tube and transformer as a composite unit securely within an enclosure, such as the cylindrical lead shielded container 22 or rectangular metal housing 23.

The resulting assembly can be slipped into a metal housing with minimum probability of voltage breakdown and minimal possibility of arcing problems between the secondary and core. Further, it will be readily evident that a structure of the kind shown in FIGS. 1 (and later in FIG. 4) is considerably smaller in size and a simpler configuration to work with while not being seriously less efficient than other types of transformer units.

Means interconnecting high voltage terminal 12 to the secondary of transformer assembly 16 includes terminal 19 formed with an interiorly threaded axial recess adapted to receive a threaded plug 24 therein. The outer end of plug 24 includes a depression, dimple or slot 26 adapted to receive the end of a high voltage conductor 27 serving to carry the high voltage from core element 18 to terminal 12.

Conductor 27 is soldered into depression 28 whereby high voltage can be transmitted between transformer l6 and tube 11. Bombardment of target assembly 13 generates considerable heat which is conducted directly to terminal 12 and at that location a heat absorbing and dissipating member 29 is carried in heat dissipating relation to terminal 12. Member 29 comprises an annular cup of suitable heat conductive and dissipating material, such as copper, soldered about the outer end of terminal 12 to be carried therefrom.

Finally, in order to minimize attenuation of the X- rays passing through the body of insulation material 21, a relieved window region 31 has been formed between a pair of radially outwardly diverging planar surfaces 32 and a bottom plane surface 33. Further, container 22 has been formed with a window 34 therein. Also, the outer housing 23 includes a window opening 36. Accordingly, any X-rays generated within tube 11 will escape substantially only in the direction of windows 34, 36.

In the formation of composite unit where a transformer will be embedded within a potting material 21 or otherwise, it is highly desirable to de-gas the transformer in order to prevent damage to the transformer in later operation. Accordingly, it is to be appreciated that with the extremely long and closely wound turns of the primary and secondary winding of the transformer coil 17, gas entrapped within the windings of the transformer will have difficulty escaping.

Accordingly, as best indicated in FIG. 2A, core 18 has been wound with primary and secondary windings 17a, 17b in a manner whereby small gaps are preserved to permit passage of entrapped gas to escape.

- Thus, longitudinally extending thin spacers 37 are first disposed about core 18 and secondary winding 17b is then wrapped about core element 18. Subsequently, spacers 38 may be disposed against the exterior winding of secondary windings 17b and, subsequently, spacers 39 are applied against spacers 37. Then, primary winding 17a can be wrapped about the unit until completed and, subsequent to completion, spacers 40 are According to another embodiment of a composite unit as disclosed herein, there is shown in FIG. 3 a composite unit 42 in which there has been combined, as in FIG. 1, a transformer and X-ray tube interconnected by means for forming an improved high voltage connection. Composite unit 42 differs from composite unit 10 in that tube 11 is formed to include a cylindrical extension 43 extending at right angles to the axis of tube 11. The outerfend of extension 43 is capped by a disc of X-ray transmissive material, such as beryllium. In this manner, even the minimal attenuation caused by potting material 21 as found in FIG. 1 can be further minimized by eliminating all the potting material defined between tube 11 and region 31.

Finally, a suitable X-ray tube, as now to be described with reference to FIG. 4, is employed in both embodiments shown in FIG. 1 and in FIG. 3, the tube being mofidied as shown by phantom lines 55 for use in FIG. 1 so as to delete extension 43.

X-ray generating tube assembly 11 comprises an elongated evacuated envelope 62 of glass or ceramic material. At one end of envelope 62 a cylindrically shaped metallic insert 63 of a type such as Kovar, suit able for forming a seal with glass or ceramic as used in envelope 62 forms a re-entrant opening 64 into which a target terminal assembly 66 can be mounted and sealed.

Assembly 66 comprises a cylindrical target in the form of the button 68 or section of suitable material, such as tungsten, to be bombarded by electrons to form X-rays. Button 68 is formed with a planar face 67 disposed transversely of the axis of button 68 and oriented at an angle on the order of 76 thereto.

Button 68 is brazed inside of insert 63 and a suitably sized anode terminal rod 12 is brazed to the back of button 68. Terminal rod 12- is preferably of a highly heat-conductive material so as to quickly dissipate heat from button 68 and form a heat sink therefor, and, accordingly, is of relatively large diameter compared to button 68.

During glassing operation, where envelope 62 is formed of glass, the thinness and poor thermal conductivity of the Kovar tubing 63 prevents heat from being excessively lost to other portions of assembly 66. At the same time, terminal 12 can be highly heat conductive to allow a rapid heat drain from button 68 during tube operation.

Terminal rod 12 is adapted to be coupled as described above to thehigh voltage core 18 of transformer 16 for handling voltages of the order of 40,000 to 150,000 volts. The other end of the lead forming the secondary 17b of coil 17 is shown as line coupled to ground.

A pulsed power supply 71 (FIG. 1) supplies short pulses of the order of 5 to 10 microseconds in duration to one end of primary coil 17a while the other end of coil 17a is coupled to ground. A cathode terminal 73, carried in the end of envelope 62 is electrically con nected to cathode assembly 74 now to be described.

A generally cylindrical metallic body 76 encloses a ceramic insulator and support plug 77. Plug 77 is held in place within body 76 by means of a retaining lip 78 bent around the rear edge of plug 77 on one side and by means of the generally conically-shaped hollow annular enclosure 79 on the other.

Coaxially of plug 77 a relatively thin, cylindrical metal thermally-insulating sleeve 81 is spot welded at its rear end to a projecting collar portion 79a of enclosure 79 and serves, at its other end, to support a heater housing 82 encircling the coils of a heater element 83. Heater 83 is connected by leads 84, 86 passing through plug 77 and, subsequently, by attachment to respective ones of the pins 87, 88. Accordingly, a rivet 89 anchors the inner end of an L-shaped support leg 91 while a connecting strap 93 or other suitable low voltage source can be used to energize the heater element 83.

The inner end of cathode assembly 74 carries an electron emitting portion adapted when charged negatively relative to the target assembly 66 to form a flow of electrons from the cathode emitting portion to the target. The electron emitting portion of cathode assembly 74 comprises an annular oxide-coated surface portion in the form of the edge of lip 94 of cup 96. Cup 96 consists of a suitable metallic material upon which there is deposited an oxide coating of a highly electron emitting material, such as strontium, calcium, or barium, or a mixture thereof, deposited around the edge of lip 94. The remainder of cup 96 can be made of conventional cathode nickel.

Cup 96 seats snugly within the cylindrical end of housing 82 directly adjacent heater element 83. A perforated triangular plate 97 is formed with a hole or circular opening encircling, in closely spaced relation, the end margin of housing 82. The outer three corners of plate 97 are secured as by suitable means such as spot welding to the tabs 98 formed on the end of body 76.

As thus arranged, the triangular plate 97 provides rigidity to the distal end of housing 76 and further serves to align and retain within relatively close tolerances the position of cup 96 with its electron emitting annular surface portions 94,

The edge of lip 94 forms a flow of electrons from cathode assembly 74 to target assembly 65. Thus, the electron emitting oxide-coated surface portion of lip 94 of the cathode is directed toward the target in a manner to form a hollow annular electron beam.

For additional focusing of the beam onto target assembly 66, a grid element 99 is carried by insulators 101, attached to plate 97 and grid 99, in coaxial relation to the oxide-coated electron emitting surface of lip 94.

Grid element 99 includes relatively broad surface portions disposed transversely to the axis of the electron beam 100 and includes tapered surface portions serving to direct the emitted electrons as a hollow beam of electrons toward the target.

Thus, grid element 99 includes an annular opening comprised of three arcuate sections 102 of a circle disposed coaxially of the center of grid element .99. Between each adjacent pair of arcuate circle sections 102 there remains a radial support 103 serving to hold a central ion shield 104 coaxially of grid 99.

Grid 99 is preferably made of refractory material whereby it will stand up under continued bombardment by ions generated within envelope 62 during operation.

As thus arranged, ions, attracted into the central portion of the electron beam 100, tend to impinge upon the shield 104 in favor of their impinging upon the elec tron emitting edge of lip 94. In this way, the life of the oxide-coated emitting edge is substantially extended.

Grid 99 influences the electron emission of beam 100 and serves to direct it as an annular hollow beam by means of the focusing action of the surfaces of grid 99 remote from cup 96. An outer annular surface 106 slopes inwardly toward the center of grid 99 at a Pierce angle, for example, on the order of 225. A second annular surface 107 tapers radially outwardly of the center of grid 99 at a similar Pierce angle. The two surfaces 106, 107 are spaced slightly to define the circular opening centrally of grid 99 through which electrons will be emitted from cathode assembly 74.

As noted above, transformer 16 is preferably operated in a pulsed mode (for reasons described below) and, upon pulsing transformer 16, target assembly 13 becomes highly positive relative to cathode assembly 74 whereby a beam current of electrons will exist. During such conductive periods, means are provided for biasing grid 99 to cathode potential or slightly above, and alsofor pulsing electron beam 100.

Accordingly, a variable resistance 108 taps into line 72 to provide a variable voltage drop between the potential on line 72 (and hence cathode 74) and the potential of grid 99. The wiper 108a is directly connected via the diode 109, poled in a direction to pass current to grid 99, via line 111 (entering envelopes 62 through one of the pins disposed through the end of envelope 62). Line 111 is directly attached to grid 99 as shown.

Accordingly, it is readily evident that grid 99 can be adjusted to the potential of cathode assembly 74 orarranged at a variable bias condition by the bias adjusting resistor 108.

Means are provided for pulsing or modulating the operation of electron beam by means of the pulse generator 112 connected directly into line 111 whereby upon application of a pulse to line 111, the electron beam 100 is gated to pass through grid 99 onwardly to target assembly 13.

In operation, assuming that heater element 83 is coupled to power supply 93 and cathode assembly 74 connected to ground via lead 72 while the core 18 is coupled to target assembly 13, an annular, hollow electron beam will be directed from the oxide-coated portions of cup 96 to impinge upon the tungsten surface of button 68. The angle of the face of button 68 relative to the axis of beam 100 serves to minimize absorption of X-raysby the anode in the region laterally to the side of the axis of beam 100. Ions created within the gas remaining in envelope 62 become positively charged and, by the attraction of beam 100, are accelerated into the interior of beam 100 whereby they seek the most negative portion of cathode assembly 74. These ions then move to bombard the ion impingement shield formed by surface 107 within beam 100 in the region of the emitting portion of cathode assembly 74. Thus, surface 107 serves to intercept ions attracted into beam 100 in favor of their striking the cathode emitting lip 94 of cup 96.

It is preferable to operate the transformer/tube assembly 10 as a composite unit using a pulsed mode so as to further protect the cathode from errosion and damage as now to be described.

It has been observed that ions are much heavier than electrons and therefore require considerably more time to move a predetermined distance than do electrons. in operating the transformer/tube assembly 10 in a pulsed mode, each pulse constitutes the application of positive high voltage to the target lasting about 5 microseconds from a suitable pulsed power supply 71 as known in the art. The 5 microsecond positive pulse is immediately followed by another pulse, almost as high, in the negative direction. There are further pulses after this, but much smaller, as the surge rapidly dampens out. The time period of 5 microseconds is substantially too short for harmful ions to reach the cathode before the negative-going pulse accelerates them in the opposite direction, thereby practically cancelling the ion motion.

Accordingly, in the operation and function of the assembly disclosed in FIGS. 1 and 5, it is preferable to operate the transformer from a pulsed power supply so as to achieve this minimizing of ion travel toward the cathode and thereby limit damage to the cathode as might be caused by ion bombardment.

The potential on grid 99 relative to the potential of the electron emitting portion 94 of cathode 74 and also the Pierce angle effect provided by the sloping surfaces 106, 107 serves to form a hollow beam 100 which can be variously adjusted so as to direct the beam 100 onto the angled surface of button 68.

The foregoing assembly has been observed to obtain a high beam current several orders of magnitude higher than those previously achieved heretofore with conventional apparatus; a long life whereby the oxide-coated cathode is not consumed or contaminated prematurely; and the beam current can be modulated relatively easily with a relatively low voltage on the order, for example, of 100 volts as distinguished from attempting to control the pulsing of the beam by controlling the high voltage potential of 40,000 to 150,000 volts applied between anode and cathode.

From the foregoing,.it will be readily evident that by providing a tube and transformer composite unit as above described, the high voltage isolation, whether it is plastic, rubber, oil or high pressure gas, does not have to be disturbed in the event of maintenance because it is much easier to change the entire assembly than to change the tube by itself. .The small size of the assembly makes this practical. In addition, no high voltage transmission line is required with its attendant difficulties and the small overall size of the assembly makes it a much more versatile X-ray source when space and weight are problems.

I claim: I

1. In combination an elongaged X-ray generating tube including a high voltage terminal, a high voltage transformer having a high voltage core element disposed proximate to said tube, means interconnecting said high-voltage terminal to said core element, means forming an enclosure around said transformer and tube, shielding means disposed about said tube and including a window therein for transmitting X-rays therethrough, and means supporting said tube and transformer lodged as composite unit within said enclosure.

2. In combination an elongated X-ray generating tube including .a high voltage terminal at one end thereof, a high voltage transformer including a core element and outer primary windings and radially inner secondary windings all disposed about said core element, said core element being disposed substantially coaxially of said high voltage terminal and tube and closely adjacent same in electrically coupled relation, means forming an enclosure around said transformer and tube including a window therein for transmitting 1 X-rays therethrough, and means, including a body of solid insulating material containing both said transformer and tube, for supporting said, tube and transformer as a composite unit securely within said enclosure.

3. The combination of an elongated X-ray generating tube including a high voltage terminal disposed axially thereof at an end thereof, a high voltage transformer having primary and secondary windings and an axially disposed core element therewithin, the primary winding of said transformer being coupled to said core element, means disposed axially of said core element and said terminal interconnectng said element and terminal, a heat absorbing and dissipating member carried in heat dissipating relation to said terminal, and means including a body of insulating material containing said transformer and X-ray tube supporting same in tandemly disposed proximate relation.

4. An X-ray tube and transformer unit according to claim 3 wherein said primary and secondary windings are mutually spaced by elongated spacers disposed radially of each other and axially along said core element, said spacers serving to provide gaps between said windings for permitting entrapped gas to escape from deep within said primary and secondary windings.

5. Apparatus according to claim 2 further including a cylindrical extension disposed to extend at substantially right angles to the axis of said elongated X-ray generating tube, and an X-ray transmissive cap disposed across said cylindrical extension for sealing same, said cap being disposed free of said body of solid insulating material.

- g Wm r (5/65) fir CEii FIFICATE OF (IQRRECTION Patent No. 66 Dated May 21, 1974 O Inventor) Nlcholas M. Gralenskl Page 1 of 3 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 7, delete "66" and substitute therefor --l3-.

line 9, delete "66" andsubstitute therefor --l3---.

line 10, delete "68" and substitute therefor 67-.

line 12, delete "68" and substitute therefor --67-.-

a line 12, delete "67" end substitute therefor 68-. line 13, delete "68" and substitute therefor 67-. line 15, delete "68" and substitute therefor 67 a line 17, delete "6 8" and substitute therefor -6 7 line 19, delete "68 and substitute therefor -67--. line 21, delete "68" and substitute therefor -67- line. 25, delete "66" and substitute therefor --l3--.

line 27, delete "68" and substitute therefor -67--. line 52, delete ",86" after "84" v i Q line 53, after "77"delete "and, subsequently, by

attachment" and substitute therefor -which subsequently lead. I

0 L line 55, delete "21" and substitu e therefor -9 E333? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. U-S- 1 66 Dated May 21, 1974 Inventor(s) Nicholas M Gralenski ge 2 of 3 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Q I Q "1 Column 4, line 56, delete "93" and substitute therefor '92 extends between leg -9 and 'pin' 87. I A six volt power supply 93.

line 61, delete "66" and substitute therefor --l3--.

Column 5, line 19, delete "65" and substitute therefor -l3.

line 24, delete "66" and substitute therefor --13-.

Column 6, line 26, delete "68" and substitute therefor -67- in both instances.

line 60, delete "5" and substitute therefor -"-4--.

Column 7, line 3, delete "68" and substitute therefor -6'.

Certain draftsman's errors appear in the drawings as follows:

In Figure 4, please change reference number "62" to --92-- for L- the strap now shown coupling pins 9 and 87;

Po-wso CERTIFICATE OF CORRECTION Patent No Dated May 21, 19-74 Inventofls) Nicholas M.

Gralenski Page 3 O'f 3 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In Figure 4,

In Figure 5,

[SEAL] in Figure 5.

add a lead line from 99 to extend to the right into grid 99 (shown in Figure 65 change reference numeral "29" and "29a" to --79 and 79a.

the lead line for reference number "87" should be drawn to the pin shown therein corresponding to pin "87" appearing in Figure 4;

lead line for reference numeral "88 should also be drawn to the pin alongside pin 87" appearing Signed and Scaled this twenty-fifth Day 'of May 1976 Attest:

C. MARSHALL DANN Commissioner oflalems and Tmdemarks RUTH c. msou Arresting Officer 

1. In combination an elongaged X-ray generating tube including a high voltage terminal, a high voltage transformer having a high voltage core element disposed proximate to said tube, means interconnecting said high voltage terminal to said core element, means forming an enclosure around said transformer and tube, shielding means disposed about said tube and including a window therein for transmitting X-rays therethrough, and means supporting said tube and transformer lodged as composite unit within said enclosure.
 2. In combination an elongated X-ray generating tube including a high voltage terminal at one end thereof, a high voltage transformer including a core element and outer primary windings and radially inner secondary windings all disposed about said core element, said core element being disposed substantially coaxially of said high voltage terminal and tube and closely adjacent same in electrically coupled relation, means forming an enclosure around said transformer and tube including a window therein for transmitting X-rays therethrough, and means, including a body of solid insulating material containing both said transformer and tube, for supporting said tube and transformer as a composite unit securely within said enclosure.
 3. The combination of an elongated X-ray generating tube including a high voltage terminal disposed axially thereof at an end thereof, a high voltage transformer having primary and secondary windings and an axially disposed core element therewithin, the primary winding of said transformer being coupled to said core element, means disposed axially of said core element and said terminal interconnectng said element and terminal, a heat absorbing and dissipating member carried in heat dissipating relation to said terminal, and means including a body of insulating material containing said transformer and X-ray tube supporting same in tandemly disposed proximate relation.
 4. An X-ray tube and transformer unit according to claim 3 wherein said primary and secondary windings are mutually spaced by elongated spacers disposed radially of each other and axially along said core element, said spacers serving to provide gaps between said windings for permitting entrapped gas to escape from deep within said primary and secondary windings.
 5. Apparatus according to claim 2 further including a cylindrical extension disposed to extend at substantially right angles to the axis of said elongated X-ray generating tube, and an X-ray transmissive cap disposed across said cylindrical extension for sealing same, said cap being disposed free of said body of solid insulating material. 